1. Field of the Invention
The present invention relates to an apparatus for measuring the weight of fluids, particularly for determining the weight of drilling, completion, and workover fluids used in drilling, completion, and workover operations in the context of oil well drilling operations.
2. Background of the Invention
The present invention relates to an improved apparatus for accurately determining the unit weight of drilling fluid being pumped into a well during drilling operations and for determining the weight of the drilling fluid returning from the well during drilling operations. Also, present invention can be used to similarily determine the unit weight of drilling fluid being utilized in completion and workover operations within the context of oil well drilling operations.
During the drilling of a well in the quest for hydrocarbons using the rotary method of drilling, it is necessary to pump or circulate a drilling fluid, known in the art as "drilling mud", downwardly through the drill pipe to which the drill bit is attached and outwardly through the drill bit into the annulus formed by the drill pipe and the wall of the well bore, for return upwardly through the annulus to the surface.
A container known as a suction tank contains the drilling fluid for pumping through the drill pipe into the well. Circulating drilling mud exits from the drill bit returning to the surface through the annulus between the drill pipe and the wall of the well bore and out into a shaker box where the cuttings which are drilled up are separated from the returning drilling fluid. Drilling fluid then flows from the shaker box via a settling tank back to the suction tank for return to the well. Drilling mud is essential to a well drilling operation as it serves to carry away the cuttings from the drill bit to facilitate drilling and act as a medium for transporting the cuttings from the drill bit area out of the well bore to be separated from the mud by means of the shaker box or settled out in a mud pit at the top of the well prior to recirculation. The primary function of the drilling mud is to act as a stopper in the well by exerting hydrostatic pressure on the bottom of the well according to the specific weight of the drilling mud thereabove to balance or overcome the formation pressure in order to prevent blowouts. The pressure of the formation adjacent to the drill bit, or bottom-hole pressure, must also be taken into consideration because this pressure must be sufficient to sustain the hydrostatic pressure of the mud in order to prevent loss of circulation, and loss of the mud as it escapes into the formation due to the pressure exerted by the mud column being substantially greater than the formation pressure itself. It is therefore essential that the pressure of the well bore, i.e., formation pressure, and the hydrostatic pressure of the drilling mud be maintained near balanced condition. The drilling mud also acts as a sealing means upon the well bore by caking on the surface of the bore to seal the bore and prevent the drilling mud from flowing out into the formation material.
It is necessary that the drilling mud be of sufficient weight to balance against the force of any upwardly acting hydrostatic pressure such as the pressure of gas, water, or oil which may be exposed in drilling and, at the same time, the drilling mud should not become so heavy that it enters the formation causing a loss of circulation. As conditions vary in the course of drilling, the weight of the drilling mud has to be altered to meet these changing conditions. For instance, if the gas sand is penetrated, the gas of the bore space will become a part of the drilling fluid. As the fluids are pumped out of the hole the gas expands, with the consequence that the mud flows out of the hole at a faster rate than it enters and the mud weight becomes considerably lighter. Such a condition must be detected immedately as remedial action may be necessary by the addition of weighted material to the drilling fluid, as otherwise the fluid might not contain the forces of the formation pressures reacting upwardly thereagainst with the consequence that a blowout may occur.
The use of excessive mud weight to provide a large factor of safety against blowouts has been previously used as a standard drilling procedure. Of course, as mentioned previously, such an overbalance may result in a loss of circulation where the formation pressures are incapable of withstanding the over-balanced hydrostatic pressures of the drilling mud. For reasons of economy, a new drilling concept of balanced pressure drilling was adopted and it became essential that continual accurate measurement of the mud weight be maintained at all times during the drilling operation, since less dense drilling mud allows faster drilling with less wear on the equipment. Since balanced pressure drilling reduced the factor of safety against blowouts, which resulted from excess mud weight, it becomes imperative that accurate and continuous mud weights into and out of the well be logged since the best evaluation of bottom hole conditions is to continuously determine the absence or to accurately measure the volume of gas entering the hole. Such a determination will provide information essential to maintaining minimal mud weight to balance the bottom hole pressures and prevent blowout conditions.
It is recognized that a continuous accurate measure of the amount of gas entering the formation is valuable information for properly balancing the well during the drilling operation. Drilling for hydrocarbons is hazardous as is evident by frequent blowouts which cause considerable air and water pollution and general ecological damage, which entails considerable expense and delays in the oil rig operation, in addition to causing periodically serious injuries. Blowouts result from the unknown relationship between formation pressures and the weight of the drilling mud which is predetermined for formation pressure containment and penetration control. The mud weight determinations are initially based on historical data for a particular formation which, of course, is only an approximation and it is therefore essential that accurate and continuous determinations be made of the drilling fluid exiting from a well since the mud weight out of the well is a reflection of the bottom hole pressure of the formation being drilled up.
The present invention is an improved apparatus for accurately measuring the weight of the drilling mud being pumped into the well and accurately determining the weight of the mud being pumped out of the well and continuously logging the information on a time chart so that an accurate determination can be made as to changes in the mud weight resulting from bottom hole conditions. The determination of mud weight in and mud weight out require two measurements, one of which is made in the suction tank which contains the drilling fluid being pumped into the well and the other measurement is made in the shaker box which contains the drilling fluid returning from the well. There have been some devices of the prior art which have attempted to make this accurate determination of the weight of the drilling mud, some of these devices of the prior art consisting of a differential pressure densiometer in which the weight of the fluid is measured by the difference in pressure between two pneumatic tubes submerged in the drilling fluid, both tubes having independently controlled air supplies, wherein the pressure of the air in each tube is regulated to the same rate and the flow is metered so that both tubes have the same volume of air at equal pressure flowing into the mud being measured, the ends of the tubes being positioned at exactly 8.35 inches apart, which represents the weight of water in pounds per gallons. Therefore, when the weight of the drilling fluid is increased or decreased, the pressure differential at the ends of the submerged tubes will increase or decrease by one inch of water pressure for each pound per gallon increase or decrease in the weight of the drilling fluid. This permits direct reading in pounds per gallon on a pressure differential recording device. However, these attempts to use pressure differential pneumatic tubes for measuring the weight of the fluid have been highly inaccurate and infeasible in actual practice.
It is the primary object of the present invention to provide an apparatus which accurately and feasibly measures the weight of the drilling mud being pumped into the well and out of the well and for continuously logging the information so that an accurate determination can be made of the changes in the mud weight resulting from bottom hole conditions.
The present invention features a single air pressure sensor tube threaded on top for mating with a connector fitting which is connected to a source of air supply and a remote pressure recorder and pressure gauge. A preferably adjustable float is mounted around the sensor tube to ensure that the sensor tube remains submerged in the fluid within the suction tank or the shaker box at a certain depth, regardless of variations in the level of the drilling fluid in the tank. A pressure gauge which is connected to the connector fitting at the top of the sensor tube above the connection of the air supply hose to the connection fitting at the top of the pressure sensor tube, measures the pressure sensed by the sensor tube in inches of water, preferably, and the remote pressure recorder connected adjacent thereto continuously records the measurements continuously generated by the pressure gauge. The air sensor tube has an open-ended bell bottom with a check valve mounted therein to prevent plugging of the sensor tube in the event the air supply is momentarily lost. The weight of fresh water is 8.34 pounds per gallon. Therefore, as in the preferred embodiment, if the sensor tube is submerged 8.34 inches in fresh water, the air pressure required to maintain air flow through the sensor tube will be 8.34 inches of water as seen on the pressure gauge and recorder. For the purposes of the present invention, this pressure is read as 8.34 pounds per gallon. In operation, the sensor tube is fixedly submersed in the drilling fluid to have its density determined, regardless of the amount of drilling fluid in the suction tank or shaker box, because of the float being fixed 8.34 inches above the bell-bottom. Further, the sensor tube is maintained in a substantially vertical position relative to the surface of the drilling fluid in the suction tank or shaker box, by means of a plurality of pulley wheels rotatably mounted on a plate which is fixably attached to a C-clamp which is securely mounted to the suction tank or shaker box, preferably, so that the sensor tube is allowed to pass between the pulley wheels when the sensor tube is vertical relative to the surface of the drilling fluid to have its density determined, the pulley wheels thereby maintaining the sensor tube in this vertical position relative to the surface of the drilling fluid. This dispositon of the sensor tube in a vertical position relative to the surface of the drilling fluid ensures an accurate reading of the density of the drilling fluid to have its density determined. Now, if the sensor tube is submersed at this 8.34 inches, but in 18.0 pounds per gallon drilling fluid, the amount of air pressure required to maintain air flow will read 18 inches of water on the pressure gauge and recorder. This pressure is interpreted as 18.0 pounds per gallon mud weight.
Because of the use of one sensor tube rather than two sensor tubes, the apparatus of the present invention has proven to achieve more accurate readings and is simpler to implement.
Many other objects and advantages of the present invention will become apparent from the detailed description of the preferred embodiment.